1. Field of the Invention
The present invention relates generally to microwave circuits and more particularly to microwave-circuit enclosures.
2. Description of the Related Art
Size, weight and cost are primary design considerations in microwave-based, electronic systems, e.g., satellite communications and phased-array radars. Accordingly, there is a continuing trend to miniaturization and integration of microwave circuits. Microwave integrated circuits (MICs) are assemblies which typically combine different circuit functions by connecting microwave devices (e.g., planar semiconductor devices and passive distributed or lumped circuit elements) with microwave transmission line structures (e.g., microstrip and stripline). MICs generally comprise two categories, hybrid MICs and monolithic MICs (MMICs).
In hybrid technology, solid state devices and passive circuit elements are carried on a dielectric, e.g., glass or ceramic, substrate. The passive circuits are typically combinations of distributed and lumped elements which are fabricated with thin and thick-film technology. In MMICs, active devices of high-frequency semiconductor systems (e.g., gallium arsenide) are grown on or in a semi-insulating substrate with various deposition processes (e.g., epitaxy, ion implantation, sputtering, evaporation, photolithography and etching) and passive elements are either deposited on the substrate or grown in it.
Although hybrid and monolithic circuits have been realized with excellent performance characteristics across a range of microwave frequencies, e.g., X, Ku and K bands, their performance has often been compromised by the microwave enclosures which shield them from associated microwave circuits. These enclosures have generally included walls and covers which form a hermetically sealed chamber. Microwave signal ports and low-frequency signal ports (e.g., for entry of control signals and DC power) are typically formed with glass encapsulated pins that transit the chamber walls.
In some cases, the pins of the microwave ports terminate on the outer side of the walls in a coaxial connector. However, microwave systems often comprise several MICs and MMICs which are positioned in close proximity. In this case, the system size is reduced by terminating the pins at each end in flat tabs which spatially correspond with the signal line of microstrip transmission lines. To allow for thermal expansion, the tabs are connected on each side of the enclosure walls to microstrip signal lines with metallic ribbons. In the wall transition region, the microwave pin is usually cylindrical and is spaced from a cylindrical bore by an annular glass seal. In the wall transition region, therefore, the pin and the wall bore form a microwave coaxial transmission line.
However, on each side of this coaxial line, the tab and attached ribbon are typically suspended in air and they form a transmission structure whose transmission parameters (e.g., phase constant, attenuation constant and characteristic impedance) are either unknown or poorly defined. Thus, a low return loss, e.g., .about.10 db, is typically realized with these conventional microwave ports (return loss is a function of incident power to reflected power). In addition, an impedance-matching network is often required between the microwave pin and the internal microwave circuits. These impedance-matching networks are time consuming to design, install and tune.
Functionally, the glass encapsulated pins act as microwave antennas and cause the enclosure's chamber to operate as a waveguide which can support a plurality of waveguide modes. This uncontrolled waveguide structure often causes instabilities in the enclosure's microwave circuits. These instabilities may be suppressed with filter networks which are positioned between stages of the microwave circuit but these networks increases the size, weight and cost of the microwave enclosure.
Conduction paths through the glass encapsulated pins are usually completed with metallic ribbons which are installed with conventional equipment, e.g., gap welders. The cost of this process is increased because support must be provided to the pins to prevent breakage of their glass seals.
Microwave enclosures with glass encapsulated pins are exemplified by part numbers 4947096-1 and 2 of Hughes Aircraft Company, the assignee of the present invention, which are manufactured by Balo Precision Parts of Butler, N.J.